1. Field of the Invention
This invention relates to a method of polarizing a material by applying an electric field thereto.
2. Description of the Prior Art
Poly(vinylidene fluoride) (PVDF) produces a piezoelectric and pyroelectric effect when polarized. This material, along with various copolymers, has been investigated for use in transducers, including microphones, headphones, loudspeakers, pressure sensors, touch-sensitive keyboards, intrusion detectors, etc. The PVDF material is most frequently formed into a film having a thickness in the range of about 6 to 150 .mu.m. In order to polarize a PVDF film to obtain a substantial piezoelectric and pyroelectric response, an electric field is applied across the film. Typical prior art methods for polarizing PVDF include thermal poling and corona poling. In thermal poling, the sample is first metallized and subjected to a field of from about 500 to 800 kV/cm at a temperature of about 90.degree. C. to 110.degree. C. over a period of typically an hour. The film is then cooled to room temperature under the applied field. In the corona technique, a corona discharge is applied to the film, typically for a few minutes' time. A summary of these techniques is given in "Piezoelectricity in polyvinylidenefluoride," by G. M. Sessler, in The Journal Of The Acoustical Society Of America, Vol. 70(6), pages 1596-1608 (1981).
More recently, an improved polarizing technique has been described by T. T. Wang and J. E. West in "Polarization of poly(vinylidene fluoride) by Application of Breakdown Fields," Journal Of Applied Physics, Vol. 53(10), pages 6552-6556 (1982). In that technique, a dielectric plate is used to form a sandwich with the film, and voltage is applied to electrodes in contact with the dielectric plate and the PVDF film. This allows for polarizing to be accomplished at lower temperatures and for shorter times, with more uniform results than was typically obtained with prior art methods. However, this technique still requires typically on the order of 20 minutes to obtain a good piezoelectric response. It is especially desirable that polarizing be accomplished at room temperature (20.degree. C.) to allow for convenient handling of the film in production, without the necessity for insertion into an oven, or contact with other heating devices. Furthermore, a reduction of the poling time is desirable, preferably to less than 1 minute.